This invention relates to a variable venturi carburetor for use with an internal combustion engine of an automotive vehicle.
As shown in FIG. 1, a conventional variable venturi carburetor for use with an internal combustion engine of an automotive vehicle includes a carburetor body 1 provided with a barrel 2 forming an air intake passage 3 therein and with a cylindrical suction chamber 4 on the barrel 2, the suction chamber 4 being closed at its one end. A lateral through-hole 2a is opened through the barrel 2 and a small diameter portion 7 of a suction piston 5 is inserted through the through-hole 2a, while a head portion 7a of the small diameter portion 7 projects into the air intake passage 3. A large diameter portion 6 of the suction piston 5 is accommodated in the suction chamber 4 and is adapted to slide on the inner circumference of the suction chamber 4. A base portion 8a of a tapered fuel metering needle 8 is attached to the head portion 7a of the small diameter portion 7 of the suction piston 5. A main nozzle 9 is mounted to a disc plate 16 which is attached to the inner surface of the barrel 2 in opposed relation with the head portion 7a of the suction piston 5. A tip portion 8b of the metering needle 8 is inserted through the main nozzle 9 and is projected into a fuel chamber 11 which is provided on the upper side of a float chamber 10. The main nozzle 9 is communicated with the fuel chamber 11 through a main jet 12. A fuel pipe 13 is attached to the lower side of the fuel chamber 11 in the vicinity of the main jet 12 and the lowermost end of the fuel pipe 13 is positioned in the fuel F stored in the float chamber 10.
There is provided a compression spring 14 in the suction chamber 4 to normally urge the suction piston 5 in the direction of the main nozzle 9, thereby defining an air chamber 15 between the outer surface of the barrel 2 and the large diameter portion 6 of the suction piston 5 when the compression spring 14 is compresssed. By the forward movement of the suction piston 5, a venturi portion 3a is defined between the head portion 7a of the small diameter portion 7 of the suction piston 5 and the nozzle mount disc 16 of the main nozzle 9, and the air intake passage 3 upstream of the venturi portion 3a is communicated with the air chamber 15 through an air hole 17. A negative pressure chamber 18 is defined between the suction chamber 4 and the large diameter portion 6 of the suction piston 5 and is communicated with the venturi portion 3a through a suction hole 19. A throttle valve 20 is provided in the air intake passage 3 just downstream of the venturi portion 3a. Reference numeral 21 indicates a float provided in the float chamber 10.
When the throttle valve 20 is opened upon operation of an engine (not shown), air is introduced through the air intake passage 3 to the engine. In particular, when the degree of opening of the throttle valve 20 is large and the velocity of air flow in the venturi portion 3a is high, air in the negative pressure chamber 18 of the suction chamber 4 is intensively sucked through the suction hole 19 into the venturi portion 3a, thereby lowering the negative pressure in the negative pressure chamber 18, while pressure in the air chamber 15 of the suction chamber 4 is kept at substantially atmospheric pressure. Accordingly, the suction piston 5 is leftwardly moved as viewed in FIG. 1 against the biasing force of the compression spring 14 and the sectional area of the venturi portion 3a is increased. The leftward movement of the suction piston 5 is accompanied by the movement of the metering needle 8 in the same direction, thereby enlarging the annular clearance between the main jet 12 and the metering needle 8 and discharging a large amount of fuel from the main nozzle 9.
When the degree of opening of the throttle valve 20 is small, such as under engine idling conditions or at low-speed running and the velocity of air flow in the venturi portion 3a is low, air in the negative pressure chamber 18 is slightly sucked through the suction hole 19 into the venturi portion 3a. Accordingly, pressure in the negative pressure chamber 18 becomes nearly atmospheric and the suction piston 5 is rightwardly moved as viewed in FIG. 1 by the biasing force of the compression spring 14, and thus the head portion 7a of the small diameter portion 7 of the suction piston 5 approaches the main nozzle 9. As a result, the sectional area of the venturi portion 3a and the annular clearance between the main jet 12 and the metering needle 8 are decreased.
In the case of low velocity of the air flow in the venturi portion 3a as is described above, fuel to be discharged from the main nozzle 9 is hardly atomized and tends to deposit on the lower surface of the metering needle 8 in the liquid state and to flow along the needle 8 toward its base portion. As shown in FIG. 4A (idling operation) and FIG. 5A (low-speed running operation), the fuel flowing along the needle 8 is dripped down from the lower surface of the needle 8 and the lower end of the head portion 7a onto the throttle valve 20 and is then supplied to the engine. Because of this intermittent dripping down of the fuel, the amount of fuel to be supplied to the engine may not be kept uniform and the air-fuel ratio of fuel mixture becomes over-rich or lean. As a result, engine speed at idling becomes unstable or surging is induced, and in the worst case, engine stall may be occurred. Upon taking off of a vehicle, the entire upper surface of the throttle valve 20 is not wet with fuel as seen in FIG. 4A and therefore the air-fuel ratio of fuel mixture temporarily becomes lean, thereby causing unsatisfactory acceleration performance from idling.